1. Field of the Invention
This invention relates to a novel catalytic combination of reactants, in particular molecular weight controllers, suitable for use with caprolactam (continuous or discontinuous) polymerization processes.
2. Prior Art
The polymer yielded by the polymerization of caprolactam (Nylon 6) in the presence of suitable additives contains about 10% of a hydroextractable material composed of caprolactam monomers and cyclic oligomers containing 2 to 7 monomeric units. Such materials must be removed from caprolactam because they considerably deteriorate the polymer characteristics, making it unusuable as a prime material for conversion into a yarn or as a man-made polymer for molding. Such materials tend to form a deposit on the extrusion dies and to surface from the extruded articles in the course of subsequent processing steps, depositing, for instance, on the spinning line rollers or creating problems of various natures during the molding step.
In particular caprolactam monomers, where the polymer is converted into a yarn or polymer porcessing temperatures in the 250.degree.-265.degree. C. range, would evaporate from the molten polymeric material and solidify over the cool areas of the system, thus producing degradation products which may discontinue the spinning process. Cyclic oligomers may readily migrate to the surface of the yarn thus produced and create a malfunction of both the rotary drive and winding members, and of the baths for subsequent dyeing, which would be highly contaminated by such products.
The extractable material, 80 to 85% of which is composed of caprolactam, is conventionally removed either by scrubbing the formed polymer with water, which process is power-consuming, or by effecting a so-called demonomerization during the polymerization itself by vacuum application. It is, however, necessary that, during the polymerization process, the polymer be so structured as to have a diminshed tendency to form caprolactam monomer and cyclic oligomers, and that the formation rate of caprolactam monomer and cyclic oligomers from the molted polymer be lower than the rate of removal of such extractable materials.
It is known that the propensity or non-propensity of the polymer to form caprolactam monomers in particular, and the reaction kinetics in general, are a function of the concentration of the terminal groups and the degree of polymerization, and that this is in turn dependent, among others, on temperature.
Past investigation work carried out by the Applicant has shown that in order to limit the formation of caprolactam, especially while spinning, the content of terminal amino-groups should be as small as possible. It has been found, for example, that in the instance of a polymer containing about 60-65 equiv/10.sup.6 g of terminal carboxylic groups, the tendency of the polymer to reform caprolactam monomers, with the polymer maintained in the molten state, has an increasing pattern up to about 40 equiv/10.sup.6 g of terminal amino-groups. Besides that concentration, the amount of reformed monomer, which is indicative of the monomer reformation rate from the molten polyamide, is maintained at a substantially constant high value.
Accordingly, in order to make the formation rate of extractable material as low as possible, which is an all-important condition for a process not providing for a water scrubbing step, it becomes necessary to synthesize a polyamide having the lowest possible content of terminal amino-groups.
On the other hand, however, the need for significant polymerization kinetics requires the provision of some terminal groups (both amino and carboxylic) because these groups enable molecular weight to be increased. Further, where the polycaprolactam is converted into a yarn, then the terminal amino-groups become necessary to impart the fiber with a good dye-taking capability.
The carboxylic groups are required to trigger the polymerization reaction, however, they also tend, albeit to a lesser degree, to promote the monomer formation where the polymer is born in the molten state.
Thus, the need arises therefrom for synthesizing a polyamide which can meet at one time the following requirements:
(a) significant polymerization kinetics, compatibly with an industrial process, and hence, with determined values of terminal, amino, and carboxylic groups;
(b) good dye-taking ability of an article obtained from the fiber resulting from that polyamide, as determined, inter alia, by the presence of amino-groups; and
(c) the rate of formation of extractable material (both monomer and cyclic dimer), with the polymer maintained in the molten state, to be the lowest possible, whereby the amount of amino groups should be as small as possible.
Since the amino and carboxylic groups of polyamide 6 vary according to the amounts and types of substances with an acidic or basic character (molecular weight controller) respectively introduced from the outset of the polymerization, by changing the relative compositions of the molecular weight controllers, one can change the content of the terminal groups.
Known in the art are many molecular weight controllers, both having an acidic nature and a basic nature, used both singly or combined together.
Known in particular is the use of mono- and di-amines having at least 6 carbon atoms and of mono- and dicarboxylic acids (U.S. Pat. No. 3,578,640), which are added individually to the reaction mass as chain terminators or molecular weight controllers in the continuous production of caprolactam having a relatively low extractable material content. Also known is to use primary or secondary amines (UK Pat. No. 1,532,603), and cyclohexylamine acetate (U.S. Pat. No. 3,477,094).
However, the molecular weight controllers used in prior polymerization processes fail to achieve at one time all of the three objectives mentioned above.
It has been now unexpectedly found that it is possible to achieve simultaneously such objects, i.e. it is possible to obtain a polyamide having significant polymerization kinetics, a high dye-taking ability, and a low content of monomer and cyclic oligomers in the molten state polymer, if a particular combination of molecular weight controllers is used, which combination affords control capabilities in a desired manner over the final product of a caprolactam polymerization process, in terms of the overall terminal amino-group content.